1. Technical Field
The present invention relates to an optical system; and more particularly, to an optical system for providing reference wavelengths.
2. Description of Related Art
There exist many applications where a very accurate measurement of the wavelength of an optical signal is required, particularly in high sensitivity sensing applications where measurement of the movement of the Bragg grating reflected wavelength by less than a picometer is required. Several different techniques and instrumentation packages have been devised which have the capacity to measure multiple wavelengths with sub-picometer accuracy, however, often these systems require a minimum of one known reference optical signal which is used to maintain the accuracy of the measurements. In systems where the wavelength range of operation is large, a set of reference signals may be required to maintain the accuracy over the entire band.
For example, FIG. 1 shows an optical signal generating scheme, which is based on the use of two broadband fiber Bragg gratings (FBGs) 6, 7 to form an etalon 8. FIGS. 1(a) and 1(b) show graphs of individual grating reflected spectrums of the Bragg gratings 6, 7 in the Bragg grating etalon shown in FIG. 1. If the gratings 6, 7 are written at the same wavelength a cavity is formed inbetween, and inside of this cavity certain multiple frequencies will resonate. The spacing between the resonating frequencies is given by the equation:
            v      F        =          c              2        ⁢        d              ,where c is the speed of light and d is an effective spacing between the gratings. Therefore, depending on the spacing between the gratings 6, 7 a comb spectrum (not shown) of narrow optical signals can be created. This comb can then provide the basis for a set of precise reference optical signals. However, the above equation only applies if the optical frequency in the cavity is within the resonant conditions of the two Bragg gratings 6, 7. If the optical frequency is outside this region, the light will pass through the cavity unaffected. In certain applications this might be a desirable feature; however, for a set of reference optical frequencies, the unaffected light is undesirable. (Compare to the technique disclosed in U.S. Pat. No. 5,892,582, hereby incorporated by reference, where two mirrors are used to form an etalon.)